On Tue, Nov 11, 2014 at 3:33 PM, Philip Reames <listmail at philipreames.com>
wrote:
> Author: reames
> Date: Tue Nov 11 17:33:19 2014
> New Revision: 221737
>
> URL: http://llvm.org/viewvc/llvm-project?rev=221737&view=rev
> Log:
> Canonicalize an assume(load != null) into !nonnull metadata
>
> We currently have two ways of informing the optimizer that the result of a
> load is never null: metadata and assume. This change converts the second in
> to the former. This avoids a need to implement optimizations using both
> forms.
>
> We should probably extend this basic idea to metadata of other forms; in
> particular, range metadata. We view is that assumes should be considered a
> "last resort" for when there isn't a more canonical way to represent
> something.
>
(naively) seems almost a pity, really - if we have the general tool, why
not drop the special cases & just use the general one? For things already
using the special cases, they can continue to look for the very specific
canonicalized case of the general form with relatively low cost, I would
imagine - but I know very little about all of this. Just a thought.
>
> Reviewed by: Hal
> Differential Revision: http://reviews.llvm.org/D5951
>
>
> Modified:
> llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp
> llvm/trunk/test/Transforms/InstCombine/assume.ll
>
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp?rev=221737&r1=221736&r2=221737&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp Tue Nov 11
> 17:33:19 2014
> @@ -1102,6 +1102,26 @@ Instruction *InstCombiner::visitCallInst
> return EraseInstFromFunction(*II);
> }
>
> + // assume( (load addr) != null ) -> add 'nonnull' metadata to load
> + // (if assume is valid at the load)
> + if (ICmpInst* ICmp = dyn_cast<ICmpInst>(IIOperand)) {
> + Value *LHS = ICmp->getOperand(0);
> + Value *RHS = ICmp->getOperand(1);
> + if (ICmpInst::ICMP_NE == ICmp->getPredicate() &&
> + isa<LoadInst>(LHS) &&
> + isa<Constant>(RHS) &&
> + RHS->getType()->isPointerTy() &&
> + cast<Constant>(RHS)->isNullValue()) {
> + LoadInst* LI = cast<LoadInst>(LHS);
> + if (isValidAssumeForContext(II, LI, DL, DT)) {
> + MDNode* MD = MDNode::get(II->getContext(), ArrayRef<Value*>());
> + LI->setMetadata(LLVMContext::MD_nonnull, MD);
> + return EraseInstFromFunction(*II);
> + }
> + }
> + // TODO: apply nonnull return attributes to calls and invokes
> + // TODO: apply range metadata for range check patterns?
> + }
> // If there is a dominating assume with the same condition as this
> one,
> // then this one is redundant, and should be removed.
> APInt KnownZero(1, 0), KnownOne(1, 0);
>
> Modified: llvm/trunk/test/Transforms/InstCombine/assume.ll
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/assume.ll?rev=221737&r1=221736&r2=221737&view=diff
>
> ==============================================================================
> --- llvm/trunk/test/Transforms/InstCombine/assume.ll (original)
> +++ llvm/trunk/test/Transforms/InstCombine/assume.ll Tue Nov 11 17:33:19
> 2014
> @@ -186,6 +186,80 @@ entry:
> ; CHECK: ret i32 0
> }
>
> +declare void @escape(i32* %a)
> +
> +; Do we canonicalize a nonnull assumption on a load into
> +; metadata form?
> +define i1 @nonnull1(i32** %a) {
> +entry:
> + %load = load i32** %a
> + %cmp = icmp ne i32* %load, null
> + tail call void @llvm.assume(i1 %cmp)
> + tail call void @escape(i32* %load)
> + %rval = icmp eq i32* %load, null
> + ret i1 %rval
> +
> +; CHECK-LABEL: @nonnull1
> +; CHECK: !nonnull
> +; CHECK-NOT: call void @llvm.assume
> +; CHECK: ret i1 false
> +}
> +
> +; Make sure the above canonicalization applies only
> +; to pointer types. Doing otherwise would be illegal.
> +define i1 @nonnull2(i32* %a) {
> +entry:
> + %load = load i32* %a
> + %cmp = icmp ne i32 %load, 0
> + tail call void @llvm.assume(i1 %cmp)
> + %rval = icmp eq i32 %load, 0
> + ret i1 %rval
> +
> +; CHECK-LABEL: @nonnull2
> +; CHECK-NOT: !nonnull
> +; CHECK: call void @llvm.assume
> +}
> +
> +; Make sure the above canonicalization does not trigger
> +; if the assume is control dependent on something else
> +define i1 @nonnull3(i32** %a, i1 %control) {
> +entry:
> + %load = load i32** %a
> + %cmp = icmp ne i32* %load, null
> + br i1 %control, label %taken, label %not_taken
> +taken:
> + tail call void @llvm.assume(i1 %cmp)
> + %rval = icmp eq i32* %load, null
> + ret i1 %rval
> +not_taken:
> + ret i1 true
> +
> +; CHECK-LABEL: @nonnull3
> +; CHECK-NOT: !nonnull
> +; CHECK: call void @llvm.assume
> +}
> +
> +; Make sure the above canonicalization does not trigger
> +; if the path from the load to the assume is potentially
> +; interrupted by an exception being thrown
> +define i1 @nonnull4(i32** %a) {
> +entry:
> + %load = load i32** %a
> + ;; This call may throw!
> + tail call void @escape(i32* %load)
> + %cmp = icmp ne i32* %load, null
> + tail call void @llvm.assume(i1 %cmp)
> + %rval = icmp eq i32* %load, null
> + ret i1 %rval
> +
> +; CHECK-LABEL: @nonnull4
> +; CHECK-NOT: !nonnull
> +; CHECK: call void @llvm.assume
> +}
> +
> +
> +
> +
> attributes #0 = { nounwind uwtable }
> attributes #1 = { nounwind }
>
>
>
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